 |
| Titel |
Watching dehydration: transient vein-shaped porosity in the oceanic mantle of the subducting Nazca slab |
| VerfasserIn |
Wasja Bloch, Timm John, Jörn Kummerow, Peter Wigger, Pablo Salazar, Serge Shapiro |
| Konferenz |
EGU General Assembly 2016
|
| Medientyp |
Artikel
|
| Sprache |
en
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250126682
|
| Publikation (Nr.) |
 EGU/EGU2016-6438.pdf |
|
|
|
|
|
| Zusammenfassung |
| Subduction zones around the world show the common pattern of a Double Seismicity Zone,
where seismicity is organized in the form of two sub-parallel planes, one at the
plate contact and the other one, 10 to 30 km below, in the mantle of the oceanic
lithosphere (Lower Seismicity Zone, LSZ). A commonly held hypothesis states that
dehydration processes and the associated mineral reactions promote the earthquakes of the
LSZ.
Fluids filling a porespace strongly alter the petropyhsical properties of a rock. Especially
the seismic P- to S-wave velocity ratio (Vp/Vs) has been shown to be sensitive to
the presence of fluid-filled porosity. It transforms uniquely to Poisson’s ratio. To
test the mineral–dehydration-hypothesis, we use local earthquake data to measure
Vp/Vs in the oceanic mantle of the subducting Nazca slab at 21∘S. We determine it
as the slope of the de-meaned differential P- vs. S-wave arrivaltimes of a dense
seismicity cluster in the LSZ. This measurement yields a value for Vp/Vs of 2.10
± 0.09, i.e. a Poisson’s ratio of ∼0.35. This value clearly exceeds the range of
Vp/Vs values expected for oceanic mantle rocks in their purely solid form at ∼50km
depth.
We follow a poroelastic approach to model the rock’s elastic properties, including Vp/Vs,
as a function of porosity and porespace-geometry. This results in a porespace model for the
target volume having a vein-like porosity occupying only a minor volume fraction. Porosity is
in the order of 0.1%. These findings are in very good agreement with field surveys and
laboratory experiments of mantle dehydration. The pore-geometry is close to the
geometrical percolation threshold, where long-ranged interconnectivity statistically
emerges, suggesting good draining capabilities. Indeed, porosity is soft so that the
amount of porosity and, consequently, permeability is very sensitive to local fluid
pressure.
We conclude that in the oceanic mantle of the subducting Nazca slab, mineral dehydration
reactions are continuously releasing water into a transient, dynamically evolving vein-system.
Permeability is most probably high enough to drain the rock at the rate of metamorphic fluid
production. |
|
|
|
|
|
|